The government's expert advisers assume that as many as 15,000 people in this country are infected with the prion infection agents that cause the lethal brain disease, variant Creutzfeldt-Jakob (vCJD), the human form of BSE or "mad cow disease". The experts don't know if this is the right figure. It could be a lot higher but ministers are refusing to fund trials of a new test to find out.

So far just 200 of our fellow citizens have developed the disease. But the prions that cause the disease can lie dormant for decades and people who are infected pose a risk to others if they are blood or organ donors, or if surgical instruments used on them are then reused on other patients.

The Medical Research Council Prion Unit at the UCL Institute of Neurology, led by Professor John Collinge, was set up at my behest to come up with ways to treat vCJD and, better still, prevent it. They were also asked to devise a test to identify vCJD in the general population. They recently announced a new blood test that does just that. This is a great breakthrough. It should enable our doctors and scientists, for the first time, to assess accurately the incidence of vCJD infection so policy decisions on how best to protect patients can be based on evidence, not guesswork.

As health secretary, the day I became aware in 1998 that it might be possible to transmit vCJD by blood and blood products, I got the experts together. Their advice was that infection through blood and blood products was likely but not certain and that the infection, if any, would probably be carried in the white blood cells. "What should we do?" I asked. The answer for the blood supply was leucodepletion (removing the white cells) at a cost of £100m. When I told Tony Blair that I'd authorised spending this £100m but hoped it would prove to be a waste of money, his response was "fuck me". The blood for a transfusion usually comes from just a few donors but blood products come from many more donors, so the chances of infection were much greater. I had to agree that we should end UK sourcing of blood products – easier said than done.

At the time these decisions had to be taken, there was no way of assessing how many people might be infected. Officials produced "computer projections", ranging from fewer than 200 vCJD fatalities to 3.5 million. These were no more than guesses. I authorised testing of tissue samples from tonsils and appendixes which eventually produced some data which was a little bit more reliable but not much. And so it has continued to this day. In their current risk assessment, the Department of Health assume that 15,000 people (one person in every 4,000) are infected but don't know.

So largely on the basis of the precautionary principle, no less than £540m has been invested since 1998 to try to protect the integrity of blood supplies and blood products. The current annual cost is over £40m plus £200m a year to supply synthetic clotting factor for the treatment of bleeding disorders.

With so many lives possibly at stake and so much money being spent, the new blood test offers the prospect of certainty for the first time. The researchers want the government to authorise trials of the new blood test so we will know how many people are likely to be infected with vCJD. Things may turn out to be better than expected or may be worse. But at least we will know and will be able to tailor the precautionary measures to the scale of the problem. That in turn may raise ethical problems about advising patients believed to be infected with vCJD until there is a treatment. Fortunately the work carried out by the Prion Unit has made such progress that doctors may soon be able to inhibit the development of vCJD, ultimately to cure it and, better still, prevent it – along with other degenerative diseases of the brain, possibly including Alzheimer's. Sadly, so far, the government have refused to authorise and fund the trials. They prefer to continue to remain in ignorance.

The health secretary Andrew Lansley has a lot on his plate at the moment, but he shouldn't let political problems divert him from his basic duty to promote the long-term interests of the nation's health. He should authorise and fund the trials of the new blood test and do it now.